nervous system
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n.
The system of cells, tissues, and organs that regulates the body's responses to internal and external stimuli. In vertebrates it consists of the brain, spinal cord, nerves, ganglia, and parts of the receptor and effector organs.
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invertebrate
vertebrate
vertebrate
(invertebrate)
All multicellular organisms have a nervous system, which may be defined as assemblages of cells specialized by their shape and function to act as the major coordinating organ of the body. Nervous tissue underlies the ability to sense the environment, to move and react to stimuli, and to generate and control all behavior of the organism. Compared to vertebrate nervous systems, invertebrate systems are somewhat simpler and can be more easily analyzed. Invertebrate nerve cells tend to be much larger and fewer in number than those of vertebrates. They are also easily accessible and less complexly organized; and they are hardy and amenable to revealing experimental manipulations. However, the rules governing the structure, chemistry, organization, and function of nervous tissue have been strongly conserved phylogenetically. Therefore, although humans and the higher vertebrates have unique behavioral and intellectual capabilities, the underlying physical-chemical principles of nerve cell activity and the strategies for organizing higher nervous systems are already present in the lower forms. Thus neuroscientists have taken advantage of the simpler nervous systems of invertebrates to acquire further understanding of those processes by which all brains function. See also Nervous system (vertebrate).
Invertebrate and vertebrate nerve cells differ more in quantity, or degree, than in qualitative features. Aside from differences in size and numbers, the most striking difference is that invertebrate neurons have a unipolar shape, whereas most vertebrate neurons are multipolar. An additional general contrast between invertebrate and vertebrate nervous systems is that invertebrates tend to have more neurons displaced to the periphery (outside the central nervous system) and to perform more integrative and processing functions in the periphery. Vertebrates perform almost all their integration within the central nervous system, using interneurons. Invertebrate nervous systems also seem to have a greater potential for regrowth, regeneration, or repair after damage than do vertebrate nerve cells. Many invertebrates continue to add new nerve cells to their ganglia with age; vertebrates, in general, do not. Only vertebrate neurons have myelin sheaths, a specialized wrapping of glial membrane around axons, increasing their conduction speed. Invertebrates tend to enhance conduction velocity by using giant axons, particularly for certain escape responses.
Nervous system (vertebrate)
A coordinating and integrating system which functions in the adaptation of an organism to its environment. An environmental stimulus causes a response in an organism when specialized structures, receptors, are excited. Excitations are conducted by nerves to effectors which act to adapt the organism to the changed conditions of the environment.
Comparative morphology
The brain of all vertebrates, including humans, consists of three basic divisions: prosencephalon, mesencephalon, and rhombencephalon (Fig. 1). The individual divisions or patterns of the brain do not function separately to bring about a final response; rather, each pattern acts on a common set of connections in the spinal cord.
Lateral views of several vertebrate brains showing evolutionary relationships.
Spinal patterns are the final common patterns used by all higher brain pathways to influence all organs of the body. These reflexes are divided into two basic patterns: the monosynaptic arc and the multisynaptic arc. The monosynaptic arc, or myotatic reflex, maintains tonus and posture in vertebrates and consists of two neurons, a sensory and a motor neuron.
The multisynaptic arc, or flexor reflex, is the pattern by which an animal withdraws a part of its body from a noxious stimulus. Both sensory neurons and internuncial neurons send information to brain centers. Coordinated limb movement is based on a connective pattern of neurons at the spinal level.
The structure of the spinal cord and its connections are basically similar among all vertebrates. The major evolutionary changes in the spinal cord have been the increased segregation of cells and fibers of a common function from cells and fibers of other functions and the increase in the length of fibers which connect brain centers with spinal centers. See also Postural equilibrium.
The rhombencephalon of the brain is subdivided into a roof, or cerebellum, and a floor, or medulla oblongata. The medulla is similar to the spinal cord and is divided into a dorsal sensory region and a ventral motor region. It is an integrating and relay area between higher brain centers and the spinal cord. In addition to these nuclei and their connections, the medulla consists of both ascending and descending pathways to and from higher brain centers. The same basic connections occur throughout vertebrates.
In mammals, the cerebellum does not initiate movement; it only times the length of muscle contractions and orders the sequence in which muscles should contract to bring about a movement. The command to initiate a movement is received from the cerebral cortex (Fig. 2). Similarly, the cerebral cortex receives information regarding limb position and state of muscular contraction to ensure that its commands can be carried out by the cerebellum.
Mammalian brain in sagittal section. Cerebellar patterns: tract 1, posterior cerebellar peduncle; 2, middle cerebellar peduncle; 3, anterior cerebellar peduncle.
The mesencephalon is divided into a roof or optic tectum and a floor or tegmentum. The tegmentum contains the nuclei of the oculomotor and trochlear cranial nerves and a rostral continuation of the sensory nucleus of the trigeminal cranial nerve.
In the evolution of vertebrates, the prosencephalon develops as two major divisions, the diencephalon and the telencephalon. The diencephalon retains the tubular form and serves as a relay and integrating center for information passing to and from the telencephalon and lower centers. The telencephalon is divided into a pair of cerebral hemispheres and an unpaired telencephalon medium.
There are three divisions of the diencephalon in all vertebrates: an epithalamus which forms the roof of the neural tube, a thalamus which forms the walls of the neural tube, and a hypothalamus which forms the floor of the neural tube. The epithalamus and hypothalamus are primarily concerned with autonomic functions such as homeostasis. The thalamus is subdivided into dorsal and ventral regions. The dorsal region relays and integrates sensory information, and the ventral thalamus relays and integrates motor information. See also Homeostasis; Instinctive behavior.
The telencephalon is the most complex brain division in vertebrates. It is divided into a roof, or pallium, and a floor, or basal region. The pallium is divided into three primary divisions: a medial PI or hippocampal division, a dorsal PII or general pallial division, and a lateral PII division, often called the pyriform pallium.
The most striking change in the telencephalon of land vertebrates involves the PIIIa component. In mammals, it has proliferated with the PIIb component of the dorsal pallium to produce the mammalian neocortex. In all land vertebrates except amphibians, the PIIb and the PIIIa components, along with the corpus striatum (BI and BII), are the highest centers for the analysis of sensory information and motor coordination. The PI, PIIa, PIIIb, BIII, and posterior parts of BI and BII form part of the limbic system which is concerned with behavioral regulation.
Comparative histology
The nervous system is composed of several basic cell types, including nerve cells called neurons, interstitial cells called neurolemma (cells of Schwann), satellite cells, oligodendroglia, and astroglia; and several connective-tissue cell types, including fibroblasts and microglia, blood vessels, and extracellular fluids.
Each neuron possesses three fundamental properties, involving specialized capacity to react to stimuli, to transmit the resulting excitation rapidly to other portions of the cell, and to influence other neurons, muscle, or glandular cells. Each neuron consists of a cell body (soma), one to several cytoplasmic processes called dendrites, and one process called an axon. Cell bodies vary from about 7 to more than 70 micrometers in diameter; each contains a nucleus and several cytoplasmic structures, including Nissl (chromophil) granules, mitochondria, and neurofibrils. The cell body is continuously synthesizing new cytoplasm, especially protein, which flows down the cell processes. The dendrites range from a fraction of a millimeter to a few millimeters in length. An axon may range from about a millimeter up to many feet in length. The site where two neurons come into contact with each other and where influences of one neuron are transmitted to the other neuron is called a synapse. Neurotransmitters are secreted across the presynaptic membrane into the synaptic cleft where they may excite (excitatory synapse) or inhibit (inhibitory synapse) the postsynaptic membrane. See also Biopotentials and ionic currents; Sensation; Synaptic transmission.
There are three layers of connective tissue membranes, the meninges, covering the brain and spinal cord: the inner, pia mater; the middle layer, the arachnoid; and the outermost, the dura mater. Between the pia mater and the arachnoid is the subarachnoid space; this space and the ventricular cavities within the brain are filled with an extracellular fluid, the cerebrospinal fluid. See also Meninges.
Comparative embryology
The anlage of the nervous system is formed in the outer germ layer, the ectoderm, although some later contributions are also obtained from the middle germ layer, the mesoderm. In most vertebrates a neural plate is formed, which later folds into a neural groove, then closes to form a neural tube. The formation of neural tissue within the ectoderm is due to inductive influences from underlying chordomesodermal structures. See also Developmental biology; Embryonic induction; Neural crest.
When the neural tube is developing, a segmentation of the central nervous system occurs by the formation of transverse bulges, neuromeres. At the time of neuromeric segmentation, the brain is subdivided into the so-called brain vesicles by local widenings of its lumen. In the rostral end more or less well-developed hemispheres are formed; in the middle of the brain anlage the mesencephalic bulge develops; and behind the latter the walls of the tube thicken into cerebellar folds. In this way the brain anlage is divided into five sections: the telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon, and its cavity is divided into the rudiments of the adult ventricles.
In spite of the extraordinary variation in adult morphology of the vertebrate brain in different species, the early phases of development are essentially similar. The spinal cord remains as a comparatively slightly differentiated tube.
The cranial or cerebral nerves are the peripheral nerves of the head that are related to the brain. Twelve pairs of cranial nerves have been distinguished in human anatomy and these nerves have been numbered rostrally to caudally as follows:
Olfactory nerve, fila olfactoria
Optic nerve, fasciculus opticus
Oculomotor nerve
Trochlear nerve
Trigeminal nerve, in most vertebrates divided into three branches: ophthalmic, maxillary, and mandibular
Abducens nerve
Facial nerve
Statoacoustic nerve
Glossopharyngeal nerve
Vagus nerve
Accessory nerve
Hypoglossal nerve
The spinal ganglia are formed from the neural crest which grows out like a continuous sheet from the dorsal margin of the neural tube and is secondarily split up into cell groups, the ganglia, by a segmentating influence from the somites. Fibers grow out from the ganglionic cells and form the sensory fibers of the spinal nerves. Motor nerve fibers emerge from cells situated in the ventral horns of the spinal cord. The ventral motor fibers and the dorsal sensory fibers fuse to form a common stem, which is again laterally divided into branches, innervating the corresponding segment of the body.
The ganglia of the sympathetic nervous system develop ventrolateral to the spinal cord as neural crest derivatives. At first a continual column of sympathetic nerve cells is formed; it later subdivides into segmental ganglia.
The parasympathetic system is made up of preganglionic fibers emanating as general visceromotor fibers from the brain and from the sacral cord segments. Cells migrate to form the peripheral ganglia along them. See also Autonomic nervous system.
| World of the Body: nervous system |
There are probably more than 100 000 million nerve cells in the body. The nervous system is the sum total of all these, together with their nerve fibres, which ramify throughout the body, and the various supporting components of nervous tissue.
The nervous system is subdivided into the central nervous system (CNS) and the peripheral nervous system (PNS). Basically, the brain and spinal cord form the CNS, while the rest is PNS. The CNS is well protected inside the skull and vertebral column. The PNS is essentially the nerves, which run through most of the tissues of the body. The function of the nervous system is to collect information from the body and the outside world, through the sense organs, to process it in the CNS, and to distribute relevant commands to the muscles and glands throughout the body.
Like other body tissues, the nervous system is composed of cells, similar in general form to other cells in the body, but with some important modifications. One might imagine that nervous tissue consists of nerve cells and very little else. In fact a multitude of other components are essential to proper functioning of the nervous system, and form an integral part of it. Still, the most important cells of the nervous system are the nerve cells (neurons). Their most distinctive feature is their thin processes, called fibres or axons, which transmit impulses (action potentials) and which contact muscles or glands, or, in most cases, other nerve cells. So the nervous system can be looked upon as an enormous series of ‘chains’ or circuits of neurons, each receiving excitatory and inhibitory messages from other neurons, and each sending impulses along its axon if the balance of incoming signals is in favour of excitation. A typical neuron in the brain may receive 10 000 terminals from incoming axons.
Many other cell types are necessary to support the neurons. Blood vessels supply blood to the nervous tissue and drain it away into the major veins. A large percentage of the human race will die from diseases associated with cerebral blood vessels, while many more people will be permanently handicapped, especially by stroke (blockage or rupture of blood vessels).
The most important — certainly the most numerous — other supporting cells in the nervous system are the glial cells, or glia (from the Greek for glue). Amazingly, there are about 10 times as many glia as neurons in the nervous system. The most distinctive glial cells in the PNS are the Schwann cells, which wrap themselves around peripheral nerves to produce the fatty, insulating sheath called myelin. In the CNS various types of glial cells are involved in myelination, the transfer of nutrients from capillaries to neurons, and are also components of the defence system of the CNS, protecting against infection and helping remove degenerated neurons.
In the PNS, groups of, usually, a few hundred axons form bundles, several of which are united into a nerve trunk. Individual axons are well protected and peripheral nerves are fairly flexible. They even stretch somewhat, which is necessary if they run near a limb joint, or when a surgeon wishes to suture together two divided nerve stumps. Larger nerves have their own tiny blood vessels.
The nervous system also includes the special sense organs (eyes, ears, etc.) and sensory axons throughout the body. The essential feature of a sense organ is the specialized neurons, called receptor cells, whose membranes include molecular mechanisms for detecting particular events outside the cell (such as the presence of particular chemicals, or light, or pressure on the membrane). Receptor cells ‘transduce’ the energy of these events into electrical changes inside the cell, which eventually produce a set of nerve impulses that race along axons towards the CNS. In the skin there are free nerve endings, specialized to signal touch, pain, and temperature. Muscle spindles and Golgi tendon organs are receptor organs found inside skeletal muscle, which are stimulated by stretch of the muscle or tension on the tendon. They help inform the CNS about the state of activity of the muscles and therefore the position and balance of the body. Such information can either be conscious (involving signals reaching the cerebral cortex of the brain) or unconscious (being used for example in spinal reflexes).
In the special sense organs, such as the eye and the ear, highly specialized receptors respond to light and sound. Sensory information also comes from the viscera and blood vessels. Although viscera can produce conscious sensation, such as pain when they are distended, visceral sensation is mainly used unconsciously by the autonomic nervous system.
The autonomic nervous system has both central and peripheral components. It is concerned with the automatic control of bodily function. It is subdivided into sympathetic and parasympathetic portions. To some extent, these two systems have opposing actions. For instance, sympathetic activity classically prepares for ‘fight or flight’, raising blood pressure and heart rate, facilitating breathing, dilating the pupils, and deviating blood from the skin and gastrointestinal tract to skeletal muscles. Parasympathetic activity, in contrast, adapts the body for rest and digestion. Cell bodies of sympathetic neurons are in the middle levels of the spinal cord. Their axons leave the cord and end on nerve cells in the sympathetic trunk, a long nerve tract beside the vertebral column. Thence the axons of these relaying nerve cells join, and are distributed with, other nerves of the peripheral system, to reach glands and blood vessels in all parts of the body (except the CNS itself). Others run to the eyes, to the heart and lungs, and to the abdominal and pelvic organs. Parasympathetic cell bodies are in the brain stem, with axons running in the tenth cranial (vagus) nerves, reaching glands around the mouth and throat, and extending down to the heart and lungs and to most of the abdominal organs. There is a second set of parasympathetic nerve cells in the lowest segments of the spinal cord, that send out fibres to the pelvic organs.
Thus the nervous system is responsible for rapid conduction of information throughout the body. Neurons are highly differentiated and, except in early fetal life, are generally incapable of division or mitosis to reproduce themselves. This means that if lost through disease or injury they cannot be replaced. On the other hand, axons regenerate readily in the PNS (as anyone who has cut a cutaneous nerve knows). One of the most important goals of neuroscience research in the years to come will be to understand why this is, and whether damaged neurons in the CNS can be persuaded to repair themselves.
Illustration
Nervous system
(Click to enlarge)
— Laurence Garey
See also autonomic nervous system; brain; central nervous system; nerves; neurotransmitter; synapse.
| Dental Dictionary: nervous system |
n
The extensive, intricate network of structures that activates, coordinates, and controls all the functions of the body. The nervous system is divided into the central nervous system, composed of the brain and spinal cord, and the peripheral nervous system, which includes the cranial nerves and the spinal nerves.
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A body system consisting of the brain, spinal cord, and nerves. It works in conjunction with the endocrine system to coordinate and direct all the activities of the body. It has a complex system of neurones that carry information in the form of nerve impulses. The nervous system provides a fast communication and coordination system between different parts of the body as well as with the outside world. See also autonomic nervous system, central nervous system, peripheral nervous system.
| Columbia Encyclopedia: nervous system |
nervous system, network of specialized tissue that controls actions and reactions of the body and its adjustment to the environment. Virtually all members of the animal kingdom have at least a rudimentary nervous system. Invertebrate animals show varying degrees of complexity in their nervous systems, but it is in the vertebrate animals (phylum Chordata, subphylum Vertebrata) that the system reaches its greatest complexity.
Anatomy and Function
In vertebrates the system has two main divisions, the central and the peripheral nervous systems. The central nervous system consists of the brain and spinal cord. Linked to these are the cranial, spinal, and autonomic nerves, which, with their branches, constitute the peripheral nervous system. The brain might be compared to a computer and its memory banks, the spinal cord to the conducting cable for the computer's input and output, and the nerves to a circuit supplying input information to the cable and transmitting the output to muscles and organs.
The nervous system is built up of nerve cells, called neurons, which are supported and protected by other cells. Of the 200 billion or so neurons making up the human nervous system, approximately half are found in the brain. From the cell body of a typical neuron extend one or more outgrowths (dendrites), threadlike structures that divide and subdivide into ever smaller branches. Another, usually longer structure called the axon also stretches from the cell body. It sometimes branches along its length but always branches at its microscopic tip. When the cell body of a neuron is chemically stimulated, it generates an impulse that passes from the axon of one neuron to the dendrite of another; the junction between axon and dendrite is called a synapse. Such impulses carry information throughout the nervous system. Electrical impulses may pass directly from axon to axon, from axon to dendrite, or from dendrite to dendrite.
So-called white matter in the central nervous system consists primarily of axons coated with light-colored myelin produced by certain neuroglial cells. Nerve cell bodies that are not coated with white matter are known as gray matter. Nonmyelinated axons that are outside the central nervous system are enclosed only in a tubelike neurilemma sheath composed of Schwann cells, which are necessary for nerve regeneration. There are regular intervals along peripheral axons where the myelin sheath is interrupted. These areas, called nodes of Ranvier, are the points between which nerve impulses, in myelinated fibers, jump, rather than pass, continuously along the fiber (as is the case in unmyelinated fibers). Transmission of impulses is faster in myelinated nerves, varying from about 3 to 300 ft (1-91 m) per sec.
Both myelinated and unmyelinated dendrites and axons are termed nerve fibers; a nerve is a bundle of nerve fibers; a cluster of nerve cell bodies (neurons) on a peripheral nerve is called a ganglion. Neurons are located either in the brain, in the spinal cord, or in peripheral ganglia. Grouped and interconnected ganglia form a plexus, or nerve center. Sensory (afferent) nerve fibers deliver impulses from receptor terminals in the skin and organs to the central nervous system via the peripheral nervous system. Motor (efferent) fibers carry impulses from the central nervous system to effector terminals in muscles and glands via the peripheral system.
The peripheral system has 12 pairs of cranial nerves: olfactory, optic, oculomotor, trochlear, trigeminal, abducent, facial, vestibulo-cochlear (formerly known as acoustic), glossopharyngeal, vagus, spinal accessory, and hypoglossal. These have their origin in the brain and primarily control the activities of structures in the head and neck. The spinal nerves arise in the spinal cord, 31 pairs radiating to either side of the body: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal.
Autonomic Nervous System
The autonomic nerve fibers form a subsidiary system that regulates the iris of the eye and the smooth-muscle action of the heart, blood vessels, glands, lungs, stomach, colon, bladder, and other visceral organs not subject to willful control. Although the autonomic nervous system's impulses originate in the central nervous system, it performs the most basic human functions more or less automatically, without conscious intervention of higher brain centers. Because it is linked to those centers, however, the autonomic system is influenced by the emotions; for example, anger can increase the rate of heartbeat. All of the fibers of the autonomic nervous system are motor channels, and their impulses arise from the nerve tissue itself, so that the organs they innervate perform more or less involuntarily and do not require stimulation to function.
Autonomic nerve fibers exit from the central nervous system as part of other peripheral nerves but branch from them to form two more subsystems: the sympathetic and parasympathetic nervous systems, the actions of which usually oppose each other. For example, sympathetic nerves cause arteries to contract while parasympathetic nerves cause them to dilate. Sympathetic impulses are conducted to the organs by two or more neurons. The cell body of the first lies within the central nervous system and that of the second in an external ganglion. Eighteen pairs of such ganglia interconnect by nerve fibers to form a double chain just outside the spine and running parallel to it. Parasympathetic impulses are also relayed by at least two neurons, but the cell body of the second generally lies near or within the target organ.
The Nervous System and Reflexes
In general, nerve function is dependent on both sensory and motor fibers, sensory stimulation evoking motor response. Even the autonomic system is activated by sensory impulses from receptors in the organ or muscle. Where especially sensitive areas or powerful stimuli are concerned, it is not always necessary for a sensory impulse to reach the brain in order to trigger motor response. A sensory neuron may link directly to a motor neuron at a synapse in the spinal cord, forming a reflex arc that performs automatically. Thus, tapping the tendon below the kneecap causes the leg to jerk involuntarily because the impulse provoked by the tap, after traveling to the spinal cord, travels directly back to the leg muscle. Such a response is called an involuntary reflex action.
Commonly, the reflex arc includes one or more connector neurons that exert a modulating effect, allowing varying degrees of response, e.g., according to whether the stimulation is strong, weak, or prolonged. Reflex arcs are often linked with other arcs by nerve fibers in the spinal cord. Consequently, a number of reflex muscle responses may be triggered simultaneously, as when a person shudders and jerks away from the touch of an insect. Links between the reflex arcs and higher centers enable the brain to identify a sensory stimulus, such as pain; to note the reflex response, such as withdrawal; and to inhibit that response, as when the arm is held steady against the prick of a hypodermic needle.
Reflex patterns are inherited rather than learned, having evolved as involuntary survival mechanisms. But voluntary actions initiated in the brain may become reflex actions through continued association of a particular stimulus with a certain result. In such cases, an alteration of impulse routes occurs that permits responses without mediation by higher nerve centers. Such responses are called conditioned reflexes, the most famous example being one of the experiments Ivan Pavlov performed with dogs. After the dogs had learned to associate the provision of food with the sound of a bell, they salivated at the sound of the bell even when food was not offered. Habit formation and much of learning are dependent on conditioned reflexes. To illustrate, the brain of a student typist must coordinate sensory impulses from both the eyes and the muscles in order to direct the fingers to particular keys. After enough repetition the fingers automatically find and strike the proper keys even if the eyes are closed. The student has "learned" to type; that is, typing has become a conditioned reflex.
Disorders of the Nervous System
A number of diseases can significantly affect the proper functioning of the nervous system. Parkinson's disease, Huntington's disease, myasthenia gravis, and amyotrophic lateral sclerosis (commonly known as Lou Gehrig's disease) are some of the more severe diseases affecting the nervous system. Strokes, which are related to circulatory disorders, also may have permanent effects on the nervous system. Certain plant derivatives, such as belladonna, cocaine, and caffeine, have a variety of stimulatory, inhibitory, and hallucinatory effects on the nervous system.
Bibliography
See D. Ottoson, Physiology of the Nervous System (1982); G. Chapouthier and J. J. Matras, The Nervous System and How It Functions (1986); L. S. Kee, Introduction to the Human Nervous System (1987); P. Nathan, The Nervous System (3d ed. 1988); J. G. Panavelas et al., ed. The Making of the Nervous System (1988).
| Biology Q&A: What is the nervous system? |
The nervous system is an intricately organized, interconnected
system of nerve cells that relays messages to and from the brain and spinal
cord of an organism in vertebrates. It receives sensory input, processes the
input, and then sends messages to the tissues and organs for an appropriate
response. In vertebrates there are two parts to the nervous system: 1) the
central nervous system, consisting of the brain and spinal cord; and 2) the
peripheral system, consisting of nerves that carry signals to and from the
central nervous system.
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| The Human Nervous System. Red is CNS and blue is PNS. | |
| Latin | systema nervosum |
The nervous system is a network of specialized cells that coordinate the actions of an animal and send signals from one part of its body to another. These cells send signals either as electrochemical waves traveling along thin fibers called axons, or as chemicals released onto other cells. The nervous system is composed of neurons and other specialized cells called glial cells (plural form glia).
In most animals the nervous system consists of two parts, central and peripheral. The central nervous system contains the brain and spinal cord. The neurons of the central nervous system are interconnected in complex arrangements and transmit electrochemical signals from one to another. The peripheral nervous system consists of sensory neurons, clusters of neurons called ganglia, and nerves connecting them to each other and to the central nervous system. Sensory neurons are activated by inputs impinging on them from outside or inside the body, and send signals that inform the central nervous system of ongoing events. Motor neurons, situated either in the central nervous system or in peripheral ganglia, connect neurons to muscles or other effector organs. The interaction of the different neurons form neural circuits that regulate an organism's perception of the world and its body and behavior.
Nervous systems are found in most multicellular animals, but vary greatly in complexity.[1] Sponges have no nervous system, although they have homologs of many genes that play crucial roles in nervous system function, and are capable of several whole-body responses, including a primitive form of locomotion. Radiata, including jellyfish, have a nervous system consisting of a simple nerve net. Bilaterian animals, which include the great majority of vertebrates and invertebrates, all have a nervous system containing a brain, spinal cord, and peripheral nerves.
Contents |
Structure
The nervous system is defined by the presence of a special type of cell, the neuron. Neurons are distinctive in a number of ways, but their most fundamental property is that they communicate with other cells via synapses, which are membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical.
All animals more advanced than sponges have a nervous system. However, even sponges, unicellular animals, and non-animals such as slime molds have cell-to-cell signalling mechanisms that are precursors to those of neurons. In radially symmetric animals such as the jellyfish and hydra, the nervous system consists of a diffuse network of isolated cells. In bilaterian animals, which make up the great majority of existing species, the nervous system has a common structure that originated early in the Cambrian period, over 500 million years ago.
Vertebrates
The nervous system of vertebrate animals is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).
Central Nervous System
Main article: Central nervous system
The central nervous system (CNS) is the largest part of the nervous system, and includes the brain and spinal cord. The spinal cavity holds and protects the spinal cord, while the head contains and protects the brain. The CNS is covered by the meninges, a three layered protective coat. The brain is also protected by the skull, and the spinal cord is also protected by the vertebrae.
Peripheral nervous system
Main article: Peripheral nervous system
The PNS is a regional term for the collective nervous structures that do not lie in the CNS. The bodies of the nerve cells lie in the CNS, either in the brain or the spinal cord, and the longer of the cellular processes of these cells, known as axons, extend through the limbs and the flesh of the torso. The large majority of the axons, which are commonly called nerves, are considered to be PNS.
The autonomic nervous system, which mediates involuntary behaviors such as heartbeat and breathing, consists of two parts: the sympathetic nervous system and the parasympathetic nervous system.
The cell bodies of afferent PNS nerves lie in the dorsal root ganglia.
Invertebrates
Neural precursors in sponges
Sponges have no cells connected to each other by synaptic junctions, that is, no neurons. They do, however, have homologs of many genes that play key roles in synaptic function. In particular, recent studies have shown that they possess a group of proteins that cluster together to form a structure resembling a postsynaptic density—the signal-receiving part of a synapse.[2] The function of this structure is currrently unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves, which mediate some simple actions such as whole-body contraction.[3]
Radiata
Jellyfish, comb jellies, and related animals have diffuse nerve nets rather than a central nervous system. In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth. The nerve nets consist of sensory neurons that pick up chemical, tactile, and visual signals, motor neurons that can activate contractions of the body wall, and intermediate neurons that detect patterns of activity in the sensory neurons and send signals to groups of motor neurons as a result. In some cases groups of intermediate neurons are clustered into discrete ganglia.[4]
The development of the nervous system in radiata is relatively unstructured. Unlike bilaterians, radiata only have two primordial cell layers, endoderm and ectoderm. Neurons are generated from a special set of ectodermal precursor cells, which also serve as precursors for every other ectodermal cell type.
Bilateria
The vast majority of existing animals are bilaterians, meaning animals with left and right sides that are approximate mirror images of each other. All bilateria are thought to have descended from a common wormlike ancestor that appeared in the Cambrian period, 550–600 million years ago. The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to anus, and a nerve cord with an enlargement (a "ganglion") for each body segment, with an especially large ganglion at the front, called the "brain".
The basic architecture of the bilaterian nervous system is seen most clearly during embryonic development. All bilaterian animals at an early stage of development form a gastrula, which is a disk with three layers of cells, an inner layer called the endoderm, which gives rise to the lining of most internal organs; a middle layer called the mesoderm, which gives rise to the bones and muscles, and an outer layer called the ectoderm, which gives rise to the skin and nervous system.[5] In vertebrates, the first sign of the nervous system is the appearance of a thin strip of cells along the center of the back, called the neural plate. The inner portion of the neural plate (along the midline) is destined to become the CNS, the outer portion the PNS. As development proceeds, a fold called the neural groove appears along the midline. This fold deepens, and then closes up at the top. At this point the future CNS appears as a cylindrical structure called the neural tube, whereas the future PNS appears as two strips of tissue called the neural crest, running lengthwise above the neural tube. The sequence of stages from neural plate to neural tube and neural crest is known as neurulation.
Worms
Planaria, a type of flatworm, have dual nerve cords running along the length of the body and merging at the tail and the mouth. These nerve cords are connected by transverse nerves like the rungs of a ladder. These transverse nerves help coordinate the two sides of the animal. Two large ganglia at the head end function similar to a simple brain. Photoreceptors on the animal's eyespots provide sensory information on light and dark.
The nervous system of the roundworm Caenorhabditis elegans has been mapped out to the cellular level. Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are known. In this species, the nervous system is sexually dimorphic; the nervous systems of the two sexes, males and hermaphrodites, have different numbers of neurons and groups of neurons that perform sex-specific functions. In C. elegans, males have exactly 383 neurons, while hermaphrodites have exactly 302 neurons [1]
Arthropods
Arthropods, such as insects and crustaceans, have a nervous system made up of a series of ganglia, connected by a ventral nerve cord made up of two parallel connectives running along the length of the belly [2]. Typically, each body segment has one ganglion on each side, though some ganglia are fused to form the brain and other large ganglia [3].
The head segment contains the brain, also known as the supraesophageal ganglion. In the insect nervous system, the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum. Immediately behind the brain is the subesophageal ganglion, which is composed of three pairs of fused ganglia. It controls the mouthparts, the salivary glands and certain muscles.
Many arthropods have well-developed sensory organs, including compound eyes for vision and antennae for olfaction and pheromone sensation. The sensory information from these organs is processed by the brain.
Microanatomy
The nervous system is, on a small scale, primarily made up of neurons. However, glial cells also play a major role.
Neurons
Main article: Neuron
Neurons are electrically excitable cells in the nervous system that process and transmit information. Neurons are the core components of the brain, the vertebrate spinal cord, the invertebrate ventral nerve cord, and the peripheral nerves. A number of different types of neurons exist: sensory neurons respond to touch, sound, light and numerous other stimuli effecting sensory organs and send signals to the spinal cord and brain, motor neurons receive signals from the brain and spinal cord and cause muscle contractions and affect glands. Interneurons connect neurons to other neurons within the brain and spinal cord.
Glial cells
Main article: Glial cell
Glial cells are non-neuronal cells that provide support and nutrition, maintain homeostasis, form myelin, and participate in signal transmission in the nervous system. In the total human brain, the number of glia is estimated to be roughly the same as neurons.[6]
Glial cells provide support and protection for neurons. They are thus known as the "glue" of the nervous system. The four main functions of glial cells are to surround neurons and hold them in place, to supply nutrients and oxygen to neurons, to insulate one neuron from another, and to destroy pathogens and remove dead neurons.
Function
At the most basic level, the function of the nervous system is to control the body. One of the most important ways it does this is by extracting information from the environment using sensory receptors, sending signals that encode this information into the central nervous system, processing the information to determine an appropriate response, and finally sending signals via the peripheral nervous system to muscles or glands, in order to activate the response.
The nervous system enables basic motor skills and sensing. The five classical senses (touch, taste, sight, smell, and hearing) are powered by the nervous system as are others such as equilibrioception (the sensing of gravity), nociception (the sensing of pain), and proprioception (the sensing of relative limb location and motion, as when touching the nose with closed eyes). Inhibition of these senses would retard basic motor skills.
Importance
The evolution of a complex nervous system makes it possible for various animal species to have advanced perception abilities like sight, complex social interactions, rapid coordination of other organ systems, and integrated processing of many concurrent signals. In humans, the advanced development of the nervous system makes it possible to have language, abstract representation of concepts, transmission of culture, and many other outcomes of human society that would not be possible without our brains.
Many people have lost basic motor skills and other skills because of spinal cord injuries. If this portion is damaged, the biggest nerve and the most important one gets damaged. This leads to paralysis or other permanent damage. Physical lesions or genetic abnormalities of the brain can also lead to major harm.
Physiological division
A less anatomical but much more functional way of dividing the human nervous system is classification according to the role that the different neural pathways play, regardless of whether or not they cross through the CNS/PNS:
The somatic nervous system is responsible for coordinating voluntary body movements (i.e. activities that are under conscious control).
The autonomic nervous system is responsible for coordinating involuntary functions, such as breathing and digestion.
In turn, these divisions of the nervous system can be further divided according to the direction in which they conduct nerve impulses:
- Afferent system by sensory neurons, which carries impulses from a somatic receptor to the CNS
- Efferent system by motor neurons, which carries impulses from the CNS to an effector
- Relay system by interneurons (also called "relay neurons"), which transmit impulses between the sensory and motor neurons (both in the CNS and PNS).
The junction between two neurons is called a synapse. There is a very narrow gap (about 20 nm in width) between the neurons called the synaptic cleft. This is where an action potential (the "message" being carried by the neurons, also known as the nerve impulse) is transmitted from one neuron to the next. This is achieved by relaying the message across the synaptic cleft using neurotransmitters, which diffuse across the gap. The neurotransmitters then bind to receptor sites on the neighboring (postsynaptic) neuron, which in turn produces its own electrical/nerve impulse. This impulse is sent to the next synapse, and the cycle repeats itself.
Nerve impulses are a change in ion balance between the inside and outside of a neuron. Because the nervous system uses a combination of electrical and chemical signals, it is incredibly fast. Although the chemical aspect of signaling is much slower than the electrical aspect, a nerve impulse is still fast enough for the reaction time to be negligible in day to day situations. Speed is a necessary characteristic in order for an organism to quickly identify the presence of danger, and thus avoid injury/death. For example, a hand touching a hot stove. If the nervous system was only comprised of chemical signals, the nervous system would not be able to signal the arm to move fast enough to escape dangerous burns. Thus, the speed of the nervous system is evolutionarily valuable, and is in fact a necessity for life.
Plasticity
To be written
Development
Main article: Neural development
Neural development in most species has many similarities with neural development in humans.
As shown in a 2008 study, one factor common to all bilateral organisms (including humans) is a family of secreted signaling molecules called neurotrophins which regulate the growth and survival of neurons[7]. Zhu et al. identified DNT1, the first neurotrophin found in flies. DNT1 shares structural similarity with all known neurotrophins and is a key factor in the fate of neurons in Drosophila. Because neurotrophins have now been identified in both vertebrate and invertebrates, this evidence suggests that neurotrophins were present in an ancestor common to bilateral organisms and may represent a common mechanism for nervous system formation.
In vertebrates, some landmarks of embryonic neural development include the birth and differentiation of neurons from stem cell precursors, the migration of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of axons from neurons and guidance of the motile growth cone through the embryo towards postsynaptic partners, the generation of synapses between these axons and their postsynaptic partners, and finally the lifelong changes in synapses which are thought to underlie learning and memory.
Pathology
Main article: Neurology
The nervous system is susceptible to malfunction in a wide variety of ways, as a result of genetic defects, physical damage due to trauma or poison, infection, or simply aging. The medical specialty of Neurology studies the causes of nervous system malfunction, and looks for interventions that can alleviate it.
The central nervous system is protected by major physical and chemical barriers. Physically, the brain and spinal cord are surrounded by tough meningeal membranes, and enclosed in the bones of the skull and spinal vertebrae, which combine to form a strong physical shield. Chemically, the brain and spinal cord are isolated by the so-called blood-brain barrier, which prevents most types of chemicals from moving from the bloodstream into the interior of the CNS. These protections make the CNS less susceptible in many ways than the PNS; the flip side, however, is that damage to the CNS tends to have more serious consequences.
Although peripheral nerves tend to lie deep under the skin except in a few places such as the elbow joint, they are still relatively exposed to physical damage, which can cause pain, loss of sensation, or loss of muscle control. Damage to nerves can also be caused by swelling or bruises at places where a nerve passes through a tight bony channel, as happens in carpal tunnel syndrome. If a peripheral nerve is completely transected, it will often regenerate, but for long nerves this process may take months to complete. In addition to physical damage, peripheral neuropathy may be caused by many other medical problems, including genetic conditions, metabolic conditions such as diabetes, inflammatory conditions such as Guillain-Barré syndrome, vitamin deficiency, infectious diseases such as leprosy or shingles, or poisoning by toxins such as heavy metals. Many cases have no cause that can be identified, and are referred to as idiopathic. It is also possible for peripheral nerves to lose function temporarily, resulting in numbness as stiffness—common causes include mechanical pressure, a drop in temperature, or chemical interactions with local anesthetic drugs such as lidocaine.
Physical damage to the spinal cord may result in loss of sensation or movement. If an injury to the spine produces nothing worse than swelling, the symptoms may be transient, but if nerve fibers in the spine are actually destroyed, the loss of function is usually permanent. Experimental studies have shown that spinal nerve fibers attempt to regrow in the same way as peripheral nerve fibers, but in the spinal cord, tissue destruction usually produces scar tissue that cannot be penetrated by the regrowing nerves.
Nervous system in humans
The human nervous system can be described both by gross anatomy, (which describes the parts that are large enough to be seen with the naked eye,) and by microanatomy, (which describes the system at a cellular level.) In gross anatomy, the nervous system can be divided into two systems: the central nervous system (CNS) and the peripheral nervous system (PNS).[8]
References
- ^ "Nervous System". Columbia Encyclopedia. Columbia University Press.
- ^ Sakarya O, Armstrong KA, Adamska M, et al. (2007). "A post-synaptic scaffold at the origin of the animal kingdom". PLoS ONE 2 (6): e506. doi:. PMID 17551586.
- ^ Jacobs DK1, Nakanishi N, Yuan D, et al. (2007). "Evolution of sensory structures in basal metazoa". Integr Comp Biol 47: 712-723. doi:. http://icb.oxfordjournals.org/cgi/content/full/47/5/712.
- ^ Ruppert EE, Fox RS, Barnes RD (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 111–124. ISBN 0030259827.
- ^ Sanes DH, Reh TH, Harris WA (2006). "Ch. 1, Neural induction". Development of the Nervous System. Elsevier Academic Press. ISBN 9780126186215.
- ^ Azevedo FA, Carvalho LR, Grinberg LT, Farfel JM, Ferretti RE, Leite RE, Jacob Filho W, Lent R, Herculano-Houzel S. (2009). Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain. J Comp Neurol. 513(5):532-41. PubMed
- ^ Zhu B, Pennack JA, McQuilton P, Forero MG, Mizuguchi K, Sutcliffe B, Gu CJ, Fenton JC, Hidalgo A (Nov 2008). "Drosophila neurotrophins reveal a common mechanism for nervous system formation". PLoS Biol 6 (11): e284. doi:. PMID 19018662. PMC 2586362. http://scivee.tv/node/8389.
- ^ Maton, Anthea; Jean Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright (1993). Human Biology and Health. Englewood Cliffs, New Jersey, USA: Prentice Hall. pp. 132–144. ISBN 0-13-981176-1.
External links
- Neuroscience for Kids
- The Human Brain Project Homepage
- Kimball's Biology Pages, CNS
- Kimball's Biology Pages, PNS
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